1. Field of the Invention
This invention relates to a control system for controlling the operation of a variable camshaft timing (VCT) system. More particularly, the invention pertains to the use of an internal differential pressure control system to reduce the axial length of an engine.
2. Description of the Related Art
Consideration of information disclosed by the following U.S. patents, which are all hereby incorporated by reference, is useful when exploring the background of the present invention.
One method to control the position of a spool valve that controls the oil flow to and from the chambers of a vane or piston style cam phaser is the externally mounted solenoid DPCS (differential pressure control system), as disclosed in U.S. Pat. No. 5,107,804.
FIG. 1 shows a spool valve (28), which is made up of a bore (31) and vented spool (25). The spool (25) is slidable to and fro within the bore (31). Phaser (60) is shown without detail in the figures. Passageways (91) to the advance and retard chamber (not shown) are shown for exemplary purposes only, and depend upon the type of phaser being used. The position of vented spool (25) within bore (31) is influenced by an externally mounted solenoid DPCS (1) that is fed by oil pressure (2) from the engine. The DPCS (1) utilizes engine oil pressure (2) to push against one end of the spool valve (28). Both ends (10) of the spool have the same area.
The DPCS (1) acts on an armature (3) of the spool (25). Pulses go to a coil (4), which actuates the solenoid (5). The area of the spool ends (10) is typically 78.5 mm2 (10 mm diameter). A control pressure that comes from either a PWM or proportional solenoid (5) pushes against the piston (6), which is typically 157 mm2 (14 mm diameter). For the externally mounted system, the solenoid and piston are mounted in front of the cam phaser. A spring (18), mounted in the phaser, pushes the spool to the default position in case of solenoid failure.
The solenoid (5) is preferably controlled by an electrical current applied to coil (4) in response to a control signal. The control signal preferably comes directly from an electronic engine control unit (ECU). The solenoid (5) can either be made to be normally open (see graph 9) or normally closed (see graph 11).
There are certain limitations with the externally mounted control system. The externally mounted differential pressure system requires the solenoid (5) and control piston (6) to be mounted either on the front cover (7) or mounted to the engine block. This increases the axial length of the engine. With either mounting method a second engine oil supply (2) has to be routed up to the solenoid control system.
The control system disclosed in both U.S. Pat. Nos. 5,172,659 and 5,184,578 utilizes hydraulic force on both ends of a spool valve. U.S. Pat. No. 5,184,578 shows the control system, in which crank and cam positions are sensed and a Pulse-width Modulated Solenoid moves a spool valve to control the actuation of the phaser, with a closed-loop control measuring the phase difference between cam and crank, and operating the spool valve accordingly.
U.S. Pat. No. 5,497,738 uses a variable force solenoid to control the phase angle using a center mounted spool valve. This type of variable force solenoid can infinitely control the position of the phaser. The force on the end of the vented spool valve located in the center of the phaser is applied by an electromechanical actuator, preferably of the variable force solenoid type, which acts directly upon the vented spool in response to an electronic signal issued from an engine control unit (“ECU”) which monitors various engine parameters.
The ECU receives signals from sensors corresponding to camshaft and crankshaft positions and utilizes this information to calculate a relative phase angle. A closed-loop feedback system which corrects for any phase angle error is preferably employed. The use of a variable force solenoid solves the problem of sluggish dynamic response. Such a device can be designed to be as fast as the mechanical response of the spool valve, and certainly much faster than the conventional (fully hydraulic) differential pressure control system. The faster response allows the use of increased closed-loop gain, making the system less sensitive to component tolerances and operating environment.